I'd mix a mild batch of any of the Oxi cleaners.
Put it directly on the stain and work in in (not necessarily a lather) and after it's soaked a while, a few minutes or so, rinse with a hose.
Inspect your progress and, if you need to, do it again.

If it's your car, after you're done have an auto tech look for and repair the oil leaks.

Try the first suggestion, then work your way down the list as necessary. Whatever you do, don't procrastinate. Bare concrete floors are porous and permanently stain if oil, grease and dirt are not removed quickly. End each remedy by hosing down and air-drying the treated area. Instructions
Difficulty: Moderate
Steps
Step One
Pour cola on the oily or dry stained areas, and leave the cola on overnight. Squirt a generous amount of dishwashing liquid into a bucket until you have a good lather. Rinse with the soapy water, then with a garden hose.

Step Two
Sprinkle baking soda or an absorbent powder such as cornmeal or sawdust on the oily spots. If the stain is dry, wet it first to make a scouring paste. Scrub with a stiff brush or push broom.

Step Three
Sprinkle automatic dishwasher detergent on the oily concrete. Leave it for several minutes, then pour boiling water on the stained area. Scrub with a stiff brush or push broom, then rinse.

Step Four
Try a commercial concrete cleaner such as Garage and Driveway Cleaner by Red Devil Co. or a grease solvent such as Benzine. Follow the manufacturer's instructions.

Step Five
Sprinkle trisodium phosphate (TSP) on the oily concrete. If the stain is dry, wet it first. Let it stand for 30 minutes. Scrub using a stiff broom. TSP is a dangerous product; if you must use it, wear rubber or latex gloves, safety goggles and protective clothing. Also, never wash a TSP product down storm drains.

Step Six
As a last resort, combat tough spills with muriatic acid and a pressure washer. Apply the acid following the manufacturer's directions, and let it soak for several seconds. Follow with a pressure washer set at 2,500 to 3,000 lbs. per square inch (psi), or 176 to 211 kg per square cm. Like TSP, muriatic acid is a dangerous product; likewise, if you must use it, wear rubber or latex gloves, safety goggles and protective clothing, and never wash such a product down storm drains.

Step Seven
After trying any of the strategies above, sprinkle baking soda over the cleaned area to neutralize the solution you've used.

Overall Tips & Warnings
Place cardboard under an oil drop or lawn mower to catch stains before they happen.
Seal concrete to prevent staining. See How to Seal a Garage Floor.
Get that leaky car fixed!
Grease solvents are flammable, so make sure you have excellent ventilation and avoid spark and flame.

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